Storage tank with inventory reduction

ABSTRACT

The disclosed floating-roof storage tank has an articulated floor with an annular outer portion, an intermediate transition portion, and a raised inner portion. Pipe elements or equipment that may hang from an outer, annular section on the roof or extend from the tank wall fit around the outside of the transition portion. Liquid-tight seams connect the transition portion of the floor to the outer portion and to the raised inner portion of the floor, sealing off an inventory-reduction compartment beneath the raised portion of the floor. The inventory-reduction compartment is supported by a solid or liquid fill or by a metal support structure. The raised inner portion of the floor may support the floating roof in a low position, and may support columns or the like that are used to support a fixed roof above the floating roof.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates generally to floating-roof storage tanksused to store liquid natural resources such as crude oil, gasoline, orthe like. The tanks may have an open top or have a fixed roof thatprovides essentially a weather shield for the floating roof.

Floating-roof storage tanks are particularly useful in the oil and gasindustry. Oil refineries and storage terminals use floating roof tanksto store liquid hydrocarbon products that have a relatively high vaporpressure, such as gasoline, napthas, and crude oil. The roof on suchtanks floats on the surface of the stored liquid, minimizing the vaporspace and thus limiting undesired vaporization of the liquid.

An ordinary 50-foot tall, 150-foot diameter tank can hold approximately148,000 barrels of liquid. However, some of the effective capacity of atraditional floating-roof tank is wasted in what is called a “heel.”

Floating-roof tanks are commonly filled and emptied in cycles. As liquidis removed from the tank, the liquid level in the tank descends. Theroof, floating on the surface of the liquid, also descends, maintaininga low-vaporization condition. However, tank appurtenances such as amixer, interior piping, or nozzles within the tank generally prevent theroof of the tank from descending all the way to the floor of the tank.Continuing to empty the tank after the roof has descended to itslowermost operating position would create a vapor space between the roofand the surface of the liquid, significantly increasing vaporization ofthe liquid and creating a potentially combustible atmosphere.Consequently, emptying operations are generally stopped when the roofreaches its lowermost operating position, and a significant volume ofliquid remains in the tank. This volume of liquid is the heel.

In an ordinary 150-foot diameter tank, the lowermost operating positionof the roof may be four or five feet above the floor of the tank,resulting in a heel of 10,000-15,000 barrels of liquid. Because thatvolume of the liquid is not emptied during normal operations, that heelrepresents a significant loss in effective working volume of the tank,and also represents a significant inventory of product not used innormal operations.

There have been efforts to address this problem. Some builders havesloped the floor of the tank. However, sloping the floor generallyeliminated only 10-30% of the heel. U.S. Pat. No. 4,957,214 proposedmounting a volume-occupying container on the bottom of the floating roofor placing a layer of ballast or relatively-dense liquid in a dam on thefloor of the tank. While this dam arrangement could provide significantheel reduction (eliminating more than 50% of the heel), it mademaintenance of the tank more difficult. For example, in order to checkfor leaks beneath the ballast, the ballast would have to be removed.

BRIEF SUMMARY

The applicant has found a solution that can provide significantly moreheel reduction than a sloped floor and can be significantly easier tomaintain than the kind of dam arrangement suggested in the '214 patent.

Like prior known devices, the new storage tank has cylindrical walls anda roof that floats on the surface of liquid stored in the tank. Unlikeprior known tanks, the new tank has an articulated floor with an annularouter portion, a raised inner portion, and an intermediate transitionportion that is narrower in width than the raised inner portion.Liquid-tight seams (for example, welded seams) connect the intermediatetransition portion of the floor to the outer portion and to the innerportion.

Generally, the raised inner portion of the floor will be at least 6″ andno more than 60″ above the outer portion of the floor. The raised innerportion will generally be made primarily of steel that is at least ⅛ ofan inch thick and no more than ¾ of an inch thick. In some embodimentsof the invention, the raised inner portion of the floor may support theroof in a low position. The inner portion of the floor may also besloped toward the intermediate transition portion of the floor.

The inclusion of the new transition portion of the floor helps tomaximize the heel reduction. In floating roof tanks, the roof may havean outer, annular section that has a bottom surface with depending pipeelements. In the new arrangement, these elements may be positioned sothat they fit around the outside of the intermediate transition portionof the floor.

Because the raised inner portion and the intermediate transition portionof the floor are sealed together, liquid stored in the tank does notenter the space beneath the raised inner portion of the floor.Consequently, when the tank is filled, the raised inner portion and theintermediate transition portion of the floor may be subjected tosignificant differential pressures. To handle these pressures, thevolume under the raised portion of the floor may be provided with asolid fill (such as a structural granular fill or concrete), with ametal support structure, or, if the outer portion of the floor is partof a continuous floor that extends beneath the raised portion of thefloor, with a liquid fill.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by referring to the accompanyingdrawings, in which:

FIGS. 1 a and 1 b are schematic side views of two floating roof tanksthat utilize the new arrangement. The tank in FIG. 1 a is an open-toptank, and the tank in FIG. 1B is a fixed-roof tank.

FIG. 2 is an enlarged, fragmentary view of the lower outer portions ofthe tank seen in FIG. 1 a.

FIG. 3 is a top plan view of the tank, with the floating roof removed.

DETAILED DESCRIPTION

The tank 10 seen in FIG. 1 a has a cylindrical wall 12, a floating roof14, and a floor 16. The tank can be built on any conventionalfoundation, such as a foundation ringwall 18 (FIG. 2), slab, berm, etc.The tank seen in FIG. 1 b is similar, but also has a fixed roof 19.

The wall 12 of the tank can vary from 10 feet to 90 feet in height, andcan be made of any suitable material. In many cases, the wall can bemade of 3/16″ to 1¾″-thick steel plates.

The floating roof 14 is positioned within the tank wall 12, and floatson the surface of liquid stored in the tank 10. The illustrated floatingroof is a pontoon roof, and the invention can also be used with otherconventional floating roofs, including pan roofs. The roof can be madeof a wide range of materials, including steel, aluminum, compositematerial, or other nonmetallic material.

The illustrated roof 14 has an outer, annular section 20 that has abottom surface 22. Elements such as a drain or a vent may hang belowthis illustrated annular section. The drain or vent can be used to drainliquid off the top of the floating roof.

As best seen in FIG. 2, the floor 16 is articulated, having an outerportion 30, an intermediate transition portion 32, and a raised innerportion 34 that are joined by liquid-tight seams 38. The variousportions of the floor can be made of conventional materials, such assteel plates or structural members.

The outer portion 30 of the illustrated floor 16 can be part of acontinuous floor that extends beneath the raised inner portion 34 of thefloor. Alternatively, the outer portion of the floor can terminate at orjust inside the lower edge of the intermediate transition portion 32 ofthe floor, as seen in FIG. 2 and explained in more detail below. Whendesired, the outer portion of the floor can incorporate a double-bottomconfiguration or an underbottom liner.

The illustrated raised inner portion 34 of the floor 16 is made of ¼″steel plate and is positioned about 36″ above the outer portion 30 ofthe floor, creating an inventory-reduction compartment 40 beneath theraised inner portion. Because the liquid-tight floor keeps stored liquidout of the inventory-reduction compartment, maintenance is simplified.

The raised inner portion 34 of the floor 16 can support the roof 14 inthe low position seen in FIG. 1. In fixed-roof tanks, such as the oneseen in FIG. 1 b, the raised inner portion can also serve as a base fora column 41 used to support the fixed roof. The illustrated innerportion of the floor slopes outwardly toward the outer portion 30 of thefloor, rising approximately 1″ every 10 feet of run. Otherconfigurations can be used. For example, the inner portion of the floorcan sometimes be as thin as ⅛ of an inch thick or as thick as ¾ of aninch thick. It can sometimes be positioned as little as 6″ or as much as5 feet above the outer portion of the floor. It can sometimes be flat orcan sometimes slope as much as 6 inches for every 10 feet of run.

The intermediate transition portion 32 of the floor 16 connects theouter portion 30 and the inner portion 34 of the floor, forming thelateral boundary of the inventory-reduction compartment 40. Theillustrated transition portion of the floor is vertical, and is spacedrelatively close to the wall 12 of the tank. Preferably, the transitionportion of the floor may be within 10 feet of the tank wall, but farenough from the tank wall to enable pipe elements, etc., that may hangdown from the annular section 20 of the roof 14 to fit around thetransition portion 32. Although other arrangements could be used, thisvertical arrangement of the transition portion of the floor optimizesthe heel-reducing volume of the inventory-reduction compartment.

The heel is the volume of the tank below the low operating level of theroof. The heel reduction provided by the invention can be measured bycomparing the heel of the product to the heel that would exist if theinventory-reduction compartment 40 were omitted, as in a conventionaltank. Above the horizontal plane 50 that extends through the raisedinner portion of the floor 20, the volume of the illustrated tank is thesame as a conventional tank. Below that plane, however, the volumesdiffer. While a conventional tank would have a heel that encompasses theentire volume below the plane 50, a tank utilizing the inventoryreduction chamber has a reduced heel volume 52 that is equal to thedifference between the corresponding volume of a conventional tank andthe volume of the inventory reduction chamber. Preferably, the volume ofthe heel reduction compartment is at least twice the volume of thereduced heel volume. This provides significant advantages overpreviously-known designs.

The transition portion 32 of the illustrated floor 16 takes the form ofa step wall that is made of steel plate or a structural member that iswelded to a step baseplate 42 on the outer portion 30 of the floor. Whenthe outer portion of the floor is part of a continuous floor, this stepbaseplate may not be necessary.

Although the articulated floor 16 offers significant advantages insimplified maintenance and heel reduction, the arrangement can present aspecial design challenge. Because the volume beneath the raised innerportion 34 of the floor is not in fluid communication with the liquidstored in the tank, the intermediate transition portion 32 of the floorand the raised inner portion 34 of the floor become subject todifferential hydraulic pressures when the tank is filled. Thus, specialsupport for the inventory-reduction compartment 40 may be needed.

The support can be provided in a variety of ways. Structural granularfill or concrete can be provided beneath the inner portion 34 of thefloor 16, as seen in the figures. A metal support structure including,for example, conventional structural members such as I-beams, can alsobe used to support the floor. Alternatively, if the outer portion 30 ofthe floor is part of a continuous floor that seals the bottom of theinventory-reduction compartment 40, the inventory-reduction compartmentcan be filled with liquid, such as water, to provide support.

This description of various embodiments of the invention has beenprovided for illustrative purposes. Revisions or modifications may beapparent to those of ordinary skill in the art without departing fromthe invention. The full scope of the invention is set forth in thefollowing claims.

1. A storage tank for liquid resources such as crude oil, gasoline, orthe like that has a cylindrical outer wall and a floor that has: anannular outer portion; a raised inner portion; an intermediatetransition portion that is narrower in width than the raised innerportion and connects the outer portion of the floor and the innerportion of the floor; and liquid-tight seams that connect the transitionportion to the outer portion of the floor and to the inner portion ofthe floor.
 2. A storage tank as recited in claim 1, in which the tankalso has a floating roof.
 3. A storage tank as recited in claim 2, inwhich the roof has a peripheral, annular section that has a bottomsurface that fits around the transition portion.
 4. A storage tank asrecited in claim 2, in which the tank has pipe elements or equipmentthat extends inwardly from the outer wall toward the intermediateportion of the floor.
 5. A storage tank as recited in claim 2, in whichthe roof has pipe elements that extend beneath a peripheral, annularsection of the roof that has a bottom surface that fits around thetransition portion of the floor.
 6. A storage tank as recited in claim2, in which the raised inner portion of the floor supports the roof in alow position.
 7. A storage tank as recited in claim 1, in whichstructural granular fill or concrete is provided beneath the raisedinner portion of the floor.
 8. A storage tank as recited in claim 1, inwhich a metal support structure is provided beneath the raised innerportion of the floor.
 9. A storage tank as recited in claim 1, in whichthe outer portion of the floor is part of a continuous floor thatextends beneath the raised inner portion of the floor.
 10. A storagetank as recited in claim 1, in which the outer portion of the floor ispart of a continuous floor that extends beneath the raised inner portionof the floor, and the tank includes a sealed, liquid-filledinventory-reduction compartment beneath the raised inner portion.
 11. Astorage tank as recited in claim 1, in which the raised inner portion ofthe floor is made primarily of steel that is at least ⅛ of an inch thickand no more than ¾ of an inch thick.
 12. A storage tank as recited inclaim 1, in which the transition portion is welded to the outer portionof the floor.
 13. A storage tank as recited in claim 1, in which theraised inner portion of the floor is at least 6″ and no more than 60″above the outer portion of the floor.
 14. A storage tank as recited inclaim 1, in which the raised inner portion of the floor is sloped towardthe transition portion of the floor.
 15. A storage tank as recited inclaim 1, in which the intermediate transition portion is approximatelyvertical.
 16. A storage tank as recited in claim 1, in which the storagetank has a diameter of at least 60 feet and the intermediate transitionportion of the floor begins to rise upwardly within 20 feet of thecylindrical outer wall of the tank.
 17. A storage tank as recited inclaim 1, in which the storage tank has a diameter of at least 60 feetand the intermediate transition portion of the floor is approximatelyvertical and is positioned within 20 feet of the cylindrical outer wallof the tank.
 18. A storage tank for liquid resources such as crude oil,gasoline, or the like that has a cylindrical wall, the tank comprising afloor that has: an annular outer portion; a raised inner portion; anapproximately vertical intermediate transition portion that connects theouter portion of the floor and the inner portion of the floor;liquid-tight seams connecting the transition portion to the outerportion and to the inner portion.
 19. A storage tank for liquidresources such as crude oil, gasoline, or the like that has acylindrical outer wall and a floor that has: an annular outer portionthat is adjacent a reduced storage volume; a raised inner portion; anintermediate transition portion that connects the outer portion of thefloor and the inner portion of the floor; an inventory-reductioncompartment beneath the raised inner portion of the floor, the volume ofthe inventory-reduction compartment being at least double a reduced heelvolume; and liquid-tight seams that connect the transition portion tothe outer portion of the floor and to the inner portion of the floor.20. A storage tank as recited in claim 19, in which the intermediatetransition portion is narrower in width than the raised inner portion.